High-power laser systems are utilized for a host of different applications, such as welding, cutting, drilling, and materials processing. Such laser systems typically include a laser emitter, the laser light from which is coupled into an optical fiber (or simply a “fiber”), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is typically engineered to produce the highest-quality laser beam, or, equivalently, the beam with the lowest beam parameter product (BPP). The BPP is the product of the laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (i.e., the beam waist, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused to a small spot, and is typically expressed in units of millimeter-milliradians (mm-mrad). A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M2, or the “beam quality factor,” which is a wavelength-independent measure of beam quality, with the “best” quality corresponding to the “lowest” beam quality factor of 1.
Wavelength beam combining (WBC) is a technique for scaling the output power and brightness from laser diode bars, stacks of diode bars, or other lasers arranged in one- or two-dimensional array. WBC methods have been developed that combine beams along one or both dimensions of an array of emitters. Typical WBC systems include multiple emitters, such as one or more diode bars, that are combined using a dispersive element (e.g., a diffraction grating) to form a multi-wavelength beam. Each emitter in the WBC system individually resonates, and is stabilized through wavelength-specific feedback from a common partially reflecting output coupler that is filtered by the dispersive element along a beam-combining dimension. Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed on Feb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat. No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107, filed on Mar. 7, 2011, the entire disclosure of each of which is incorporated by reference herein.
Ideally, a WBC laser system with a diffractive grating combines multiple emitters at different wavelengths individually defined by the grating into a single output beam of multiple wavelengths with a beam quality comparable to single emitter. However, in practice, the beam quality of the combined output beam may be significantly worse than that of a single emitter. One major reason is that different beams from different emitters have different incident angles on the grating, which causes different projected beam sizes on the grating. In such cases, the beams may not overlap each other completely on and optically downstream of the grating. The different angles of incidence may also result in severe dislocations of waists of diffracted beams and therefore reduce feedback uniformity and WBC resonator efficiency.
In addition, since a WBC system requires different emitters operating at different wavelengths, the available wavelength bandwidth of the source array may be a critical resource. In many cases, this bandwidth sets the requirement for dispersion power of the grating and defines the maximum number of combinable emitters or the minimum size of a WBC resonator. Reduction in the usage of wavelength bandwidth is equivalent to increasing dispersion power, and also may help improve laser performance, such as faster cold-start, higher efficiency, etc.